1. Field of the invention
This invention is directed to plugs for use in well operations, and in certain embodiments is particularly directed to cementing and testing operations.
2. Description of the Prior Art
Cementation is one of the most critical processes in drilling and completing a well and it is an important part of the running of casing. Cementing is done at various points in the well and at various times while drilling both inside and outside of the casing.
The primary cementing can form a protective sheath around the casing, segregating producing formations to prevent migration of undesirable fluids. Secondary cementing takes place after the primary cementing and can be used to squeeze cement into the perforations in the casing or to seal off, isolate or repair parts of the well. Plug back cementing is used to place cement at desired points in the well or to shut off the bottom water or reduce the depth of the well.
Two of the apparatuses, pipe attachments, or "float equipment" routinely used in the cementing operations are the collar and the shoe. These are typically cement restrictions or shoulders which are attached to a pipe string as a part of the pipe string. The collar, for example a float collar, is inserted between the top and bottom of a casing string usually one or two joints above a float shoe which is attached to the bottom of a pipe string. Shoes and collars, among a number of things, help prevent the back flow of cement during the cementing operation. The collars and the shoes are usually equipped with a check valve to aid in the prevention of back flow of cement. The shoes and collars are typically an outer cylindrical housing or pipe and an inner cement tube communicating with and fixed to the inner surface of the cylindrical housing, with a fluid passage running the length of the cement tube. When there is a check valve, it is usually part of an inner housing in concentric spaced relationship with the outer housing so that the cement tube fills the space between the two housings and the inner housing forms part of the fluid passage.
In addition to the collars and shoes typical cementing operations employ one or more pump down plugs. Pump down wipe plugs can serve a variety of purposes: (1) to separate or serve as the interface between the cement slurry from the fluid it is displacing or the fluid which is being used to displace the cement to the desired level; (2) to wipe off the inner surface of the pipe string as it passes; (3) to help prevent back flow while the cement is setting up; and (4) as a sealing means for checking the integrity of casing strings with high hydrostatic pressure.
In practice the well operator makes up his pipe string so that the collar or shoe is lowered into the well to the desired level. When he decides to cement he may place a bottom pump down plug between the fluid already in the well and the wet cement. This bottom plug has a fluid passage through it which is sealed by a diaphragm or membrane. The cement is pumped into the well forcing the bottom plug down the well, displacing. the fluid in front of it, until it reaches the top of the cement tube of the shoe or collar or shoulder. This restriction stops the plug and increased pumping pressure breaks the diaphragm or membrane and the cement passes through the plug and through the fluid passage of the collar or shoe. After the desired amount of cement is pumped into the well a top pump down plug is inserted to act as the interface between the fluid used to force the cement to the desired level. Often the bottom plug is not used and only one plug as the interface between the cement and the fluid is used to force the cement to the desired level. The top plug is usually pumped until it comes in contact with the bottom plug if one is used or the top of the cement tube part of the shoe or collar. The cement is allowed to set or harden and the well operator then carries out whatever other operations he intends to do.
The prior art plugs are usually made of a pliable or rubbery material, such as plastic, wood or rubber, sometimes with hollow metal or plastic cores and they fit snugly in the pipe string.
Once the cement has set up and the well operator has carried out his desired operations he may decide to drill out the plug and/or plugs, collar or shoe and the cement. The plugs are typically made of drillable material, as are the cement tube and innerhousing of the collar and shoe and of course, the cement which was pumped into the well. The well operator lowers the drill string into the well until the drill bit contacts the plug and he begins to drill by rotating the drill bit, usually clockwise.
Stage cementing tools are used to protect formations or when an extremely long column of cement is required. A common practice is for the tool or tools to be positioned in the string as it is run. During the cementing process an operating tool called a "bomb" is dropped. The bomb contacts a projection in the tool and on application of pressure causes the tool to open and cement is pumped into the annulus. Upon completion of cement displacement an operating tool called a "closing plug" is dropped. The closing plug contacts a projection in the tool and on application of pressure causes the tool to close. On completion of cementing, the stage tool projections, bomb and closing plug are drilled out.
It is sometimes desirable to pressure test the string with the closing plug in place. When done, a hydrostatic pressure is applied on top of the closing plug resulting in substantial loading.
Often during drill out, problems are encountered because of the elastomeric nature of the material used in many plugs; i.e., drilling is made more difficult because a drill bit may seize or "grab" the material rather than drilling through it easily as it does through taut, hardened materials such as cement. Other problems with plugs are related to the fact that, although no two wells are identical, the available plugs are fairly standardized stock items which do not permit customization for a specific job. Customized plugs for each job would be prohibitively expensive. It is also expensive, however, to maintain an inventory of the variety of available standard plugs. For example, although a piece of casing may be nominally designated as "7 inch casing", inside diameters of such casing can vary significantly depending on the weight per foot of the casing; e.g., 7 inch casing of 20 pounds per foot can have an inside diameter of 6.456 inches and a wall thickness of 0.272 inches, while that of 38 pounds per foot will have an inside diameter of 5.920 inches and a wall thickness of 0.540 inches.
To solve problem of drilling materials like rubber, prior art methods have included the use of a burlap sack full of glass bottles thrown down the well onto the plug. Previous attempts to solve the problems related to the non-availability of customized plugs include maintaining a large inventory of various different standard plugs.
There has long been a need for an easily drillable plug for well operations. There has long been a need for a plug whose physical dimensions (length, width, diameter) and physical properties can be readily and quickly customized for a particular job. There has long been a need for a plug which has a reduced amount of elastomeric material, but which can effectively operate within the well. There is an unsatisfied demand for a plug which uses less elastomeric material. There has long been a need for a plug which provides an effective seal within a well. There has long been a need for a plug which provides an effective flow path for fluid between a plate and a plug or between two plugs so that there is no unwanted pressure build-up of fluid between these items.